The present invention relates generally to a thermoplastic vulcanizate, and more particularly, to a thermoplastic vulcanizate suitable for use as a membrane for covering roofs.
Polymeric sheeting material has been used for several years in the roofing and construction industry for covering industrial and commercial flat roofs of new buildings and for refurbishing the roofs of older buildings. Typically, such sheeting material is generally applied to a roof surface in the form of a single ply of roof sheeting material. Alternatively, a double-ply roof sheet, often referred to in the industry as a “bilaminate,” may be used. Whether single- or double-ply, such roofing materials for covering large buildings are customarily available in sheets having a length up to several hundred feet and a width of from about 3 feet to about 50 feet.
During installation, a number of edge-joining methods can be employed. For example, the roofing membranes may be primed with a liquid-applied splice adhesive at their overlapped edges and the primed overlapped edges joined together to form a weather-impermeable sealing barrier. Alternatively, a splicing tape has been used. In the case of other roofing membranes, adjacent margins of adjacent sheets are solvent-welded or heat-welded in overlapping relationship to form a sealing membrane.
Generally, roofing applications require a roofing membrane to be very flexible in order to precisely conform to the contours of the deck of the roof to which it must be affixed. Yet, the membrane must be resilient enough to protect the roof to which it is applied. To that end, elastic properties of the polymeric sheeting can be important as they can affect the dimensional stability and tear resistance of a roofing membrane over time. To perform as an effective roofing membrane, it is preferred that the ultimate tensile of elongation of the membrane after thermal aging not decrease significantly. Such a requirement means that the membrane must also have a high tensile strength. Effective performance of the membrane further requires that the high tensile strength be accomplished without sacrificing the other properties that make a resin suitable or desirable for production of a roofing membrane such as burn resistance, ultraviolet degradation resistance and ready processability, as well as aesthetics.
Thermoset materials such as ethylene-propylene copolymers (EPM) and ethylene-propylene-diene terpolymers (EPDM) have been extensively made into polymeric sheeting for use as roofing membranes. EPM and EPDM sheeting materials usually are prepared by compounding the EPM, EPDM or mixtures thereof with the appropriate fillers (e.g., carbon black), processing materials, curing additives and other optional ingredients, in a suitable mixer and calendering the resulting compound into a sheet of desired dimensions.
While curing imparts beneficial properties to roofing membranes, a disadvantage of thermoset compositions (e.g., EPM and EPDM) is the difficulty with which successive sheets of these elastomers adhere to one another. This can be a serious problem, because in applying elastomeric sheets to a roof, they are usually spliced or otherwise attached at their edges in order to form a continuous watertight barrier. These splice or seam areas are subjected to both short-term and long-term stresses such as those caused by roof movement, heavy winds, freeze-thaw cycling and thermal cycling. Such stresses may manifest themselves in shear forces or peel forces. For example, a lapped seam or edge may peel back and separate under severe stress conditions, resulting in a half-cylindrical or half-conical opening or void in the lapped seam or edge (known as a “fishmouth” or “edge wrinkle” condition) under less severe conditions.
One solution to this problem has been the use of thermoplastic materials. Polyvinyl chloride or “PVC” roofing membrane sheets have been used since the 1960s. Plasticized PVC compositions meet the high tensile strength service requirement of roofing membranes, while also retaining other properties required for service as a roofing membrane. Plasticized PVC compositions, however, have not met all of the needs of a flexible membrane. The plasticizer required to impart processability to the PVC resin so that it may be fabricated into a membrane, tends over time to migrate to the surface of the roofing membrane upon exposure to ultraviolet light. Having a high surface area, the membrane loses plasticizer over its service lifetime, thereby promoting hairline cracks, which in time can lead to the eventual failure of the PVC roofing membrane. Additionally, the chloride content of PVC membranes poses an environmental hazard.
In more recent years, thermoplastic elastomers or “TPEs” have been used as a solution to adhesion problems experienced with thermoset compositions. The typical TPE structure is that of a matrix of a hard, plastic component with discrete domains of a softer, elastomeric component embedded therein. This structure yields a composite having a behavior similar to a cured elastomer, but with the advantage that it undergoes plastic flow above the softening point of the plastic component. This permits TPEs to be fabricated into thin sheets via calendering.
When both the plastic and elastomeric components of a TPE are at least predominantly olefinic, the resulting polymer is referred to as a thermoplastic olefin or “TPO.” A typical TPO is a melt blend or a reactor blend of a polyolefin plastic, typically polypropylene, with an olefin copolymer elastomer (OCE), typically an ethylene-propylene rubber. The polyolefin plastic imparts to the TPO superior melt flowability, while the olefin copolymer elastomer provides flexibility.
Nonetheless, TPOs have been used in the prior art roofing membranes with only limited success. The elastic properties of TPOs are specifically an issue because TPOs generally cannot be folded and have a non-recoverable deformation nature. Furthermore, when successive sheets of a TPO roofing membrane are applied to a flat roof surface and spliced together side by side, shrinkage with the help of reinforced scrim may result thereafter pulling the seams apart and resulting in a leaky roof. Also, the fluid-like melt of TPOs makes weld seams difficult to control, making scrim support necessary to increase the dimensional stability of the roofing membrane. Additionally, some TPO membranes, particularly black TPO sheets which absorb more heat, shift in weather extremes, becoming baggy in the summer and taut to the point of straining in the winter. As to processability (calendering) of TPOs, the low melt strength of TPOs may cause high neck down, with gauge being highly dominated by line speed. Laminated sheet may be highly oriented in the machine direction resulting in low tear in machine direction.
Conventional elastomeric roof-sheeting materials have also proven problematic as they are susceptible to burning when exposed to open flame. Enhanced flame resistance can be imparted to a small extent by use of olefinic elastomers, fillers such as antimony trioxide, decabromo diphenyl oxide (DBDPO), dechlorane (chlorinated alicyclic hydrocarbon) alumina trihydrate, and chlorinated or brominated paraffins, and the like, which can be incorporated into the composition. However, the capacity of such roof-sheeting membranes to accept flame retardant fillers is limited, typically on the order of about 5 to about 15 parts by weight flame retardant fillers per 100 parts by weight rubber, especially with membranes which exhibit thermoplastic characteristics.
Also, fully cured thermoset EPM/EPDM is traditionally reinforced with carbon black. A black color is generally imparted to conventional roofing membranes because of the use of carbon black, as well as coal filler and related petroleum-derived materials or other similarly dark-colored fillers, in the composition from which the sheeting material is prepared. While having other desirable properties, a black roofing sheet can have the effect of raising the interior temperature of a building as it readily absorbs radiant energy from the sun. Accordingly, various attempts have been made to provide non-black and, more specifically, mineral-filled, white roof sheeting compositions with no sacrifice in the aforementioned desirable properties.
White rubber membrane compositions have been typically produced from a variety of the polymeric rubbers listed hereinabove and non-black mineral fillers such as soft and hard clays, chemically modified clays, calcium carbonate, silicas, talc, mica or titanium dioxide, and non-staining process oils. While these white rubber membranes can be used by themselves on a roof, they are more often laminated to standard black rubber membranes which are well known in the art. Typically, this is done by separately extruding and preferably calendering the white rubber membrane compounds and the black rubber membrane compounds and then laminating them together by either a calendering or coextrusion process while the membranes are still in the uncured state. Thus, the black and white rubber plies which comprise the bilaminate are knitted together during the curing process, the mineral filled, white-colored roof sheeting material preferably serving as the top ply and the carbon black-reinforced sheeting material serving as the bottom ply of the membrane bilaminate.
Unfortunately, attempts at fashioning a non-black roofing sheet have proven a challenge to those skilled in the art. Previous attempts to fashion a non-black roofing sheet of any composition have resulted in sheets lacking the essential properties of long term weathering, burn resistance and ready processability.